An evaluation of nitroblue tetrazolium dye reduction and hexose monophosphate shunt activity of white blood cells has been undertaken in selected groups of children. Study groups include: (1) premature and full term (normal and infected) neonates; (2) children with sickle cell disease or other hemoglobinopathies during periods of health, crisis and during infection; (3) study of children with acute lymphocytic leukemia and with solid tumors during remission, relapse and infection; and (4) a 3 year, double blind study of the value of the NBT dye test in differentiating acute from chronic infection in patients with cystic fibrosis. In addition, a study of factors affecting NBT dye reduction in vitro has been undertaken. To date, these parameters have been evaluated in 114 healthy full term infants, 16 infected full term infants, 20 healthy premature infants and 9 infected premature infants. These studies also have been performed upon blood obtained upon 325 occasions from 93 patients with acute lymphocytic leukemia or solid tumors during periods of remission andrelapse, and in the presence and absence of infection. Sixteen patients with SS or SC disease have been evaluated during periods of good health and 7 patients with SS or SC disease have been sampled during periods of infection.